Flat-panel displays including liquid crystal displays and plasma displays are applied to a wide variety of apparatuses ranging from mobile apparatuses such as mobile telephone terminals to large-size apparatuses such as public displays placed in streets. Many of such displays are developed with emphasis on realizing a wide angle of visibility, high brightness and high image quality, and have been required to display contents beautifully and so as to be easy to view from any angle.
On the other hand, contents displayed on displays include ones such as confidential information and private data that should not be seen by others. Therefore, at the present time when progress is being made toward ubiquitousness with the development of information apparatuses, preventing displayed contents from being seen by others even in public space where there are unspecified people is an important problem.
In addition, even in a place such as an office where there are only specified people, there are cases where confidential information is handled that should not be seen by persons who pass behind the seat.
Some of the mobile telephone terminals and the like are provided with a display having an optical shielding plate or a louver film_so that the display contents can be visually recognized only from a specific direction. However, since the display contents can be furtively seen from directly behind the user, this structure is not sufficient from the viewpoint of the preservation of confidentiality.
A technology related to solving these problems is an “image display apparatus” disclosed in Patent Document 1. This image display apparatus is an apparatus that enables, by making the user wear glasses having an image selection function, only a person (user) wearing the glasses to visually recognize a specific image (hereinafter, referred to as confidential image) and presents a different image (hereinafter, referred to as public image) to other persons.
Specifically, in the image display apparatus shown in FIG. 1, one frame of input image signal 11 is stored in an image information storage memory 12 based on a frame signal 13. Then, the image information is read from the memory 12 at a speed twice the frame period (that is, read twice during one frame period), the signal that is read first is compressed to ½ and inputted to a synthesis circuit 15 as a first image signal 14, and the image signal that is read next has its chroma and brightness converted by a chroma and brightness conversion circuit 16 and inputted to the synthesis circuit 15 as a second image signal 17. Consequently, the first image signal 14 and the second image signal 17 are alternately displayed on an image display 18.
On the other hand, the frame signal 13 is also inputted to a glass shutter timing generation circuit 19. The glass shutter timing generation circuit 19 drives the shutter of glasses 21, and controls the glass shutter so that the image by the second image signal 17 is not seen by the user.
With such a structure and operation, persons not wearing the glasses 21 see a gray image which is a synthetic image of the first image signal 14 and the second image signal 17 and which is not related to the first image signal 14, or a third image (public image), and persons wearing the glasses 21 see a desired image (confidential image) based on the first image signal 14.
With a similar structure, a plurality of persons can selectively view different images with one display apparatus. A technology related thereto is a “television multiple display system” disclosed in Patent Document 2.
In the television multiple display system disclosed in Patent Document 2, a plurality of programs are sequentially shown within the afterimage remaining time in a time-sharing manner on one image display screen and glasses having a shutter function are caused to operate in time-sharing cycles, whereby a plurality of viewers can simultaneously watch desired programs independently of each other.
In the inventions disclosed in Patent Document 1 and Patent Document 2, there is a problem in that a person viewing a confidential image or a desired program by using the shutter glasses can view another confidential image or program displayed on the same display apparatus by changing the phase of the opening and closing timing of the shutter glasses (see FIG. 2 of Patent Document 1 and FIG. 5 of Patent Document 2).
There is also a problem in that when the number of confidential images or programs is increased, the period during which the shutter glasses are open (ON period) is decreased in inverse proportion to the number of images or the number of programs and this makes dark the confidential image or the program viewed through the shutter glasses.
Another related technology for solving the above problem of preventing the displayed contents from being seen by others even in an environment where there are unspecified people is a “method of providing data that can be privately viewed on a display that can be viewed by the public” disclosed in Patent Document 3. According to the method disclosed in Patent Document 3, only authorized users are enabled to make out the private image (confidential image) on the display, and at the same time, unauthorized users are made to see, as the public image, merely a random pattern or a pattern that is difficult to make out, or an image such as a screen saver image.
To promote this purpose, according to the invention disclosed in Patent Document 3, an image processing technique including a data concealment pattern and an alternation pattern is synchronized with a display incorporating an image created by an image processing technology (for example, combined with a wearable device such as active glasses). Lastly, by the “known ability to fuse dissimilar images into a single image” of the human visual system, the capability is completed to provide data that can be privately seen on a display that can be seen by the public.
For the problem in that a person viewing a confidential image or a desired program by using the shutter glasses can view another confidential image or program by changing the phase of the opening and closing timing of the shutter glasses, by randomizing the opening and closing timing of the shutter by using a (pseudo) random number generator, the invention disclosed by Patent Document 3 can make it substantially impossible to completely view another confidential image or program even when shutter glasses in which the frequency and the phase can be changed are used.
Patent Document 1: Japanese Patent Application Publication Laid-Open No. S63-312788
Patent Document 2: Japanese Patent Application Publication Laid-Open No. S62-65580
Patent Document 3: Japanese Patent Application Publication Laid-Open No. 2001-255844